The present invention relates to engine cooling systems.
Generally, a water cooling type engine of a vehicle includes a cooling system provided with a radiator and a flow control valve. The radiator is located in an engine coolant circuit for cooling the coolant. The flow control valve regulates the flow of the coolant that passes through the radiator. The flow control valve is controlled to change the coolant flow in the radiator (hereafter, “the radiator flow”). This adjusts the temperature of the coolant, which cools the engine.
For example, Japanese Laid-Open Patent No. 10-317965 describes a known control procedure of the flow control valve. According to the procedure, the flow control valve is fully closed to minimize the radiator flow when the coolant temperature is relatively low. In contrast, when the coolant temperature is relatively high, the flow control valve is fully opened to maximize the radiator flow. Otherwise, a feedback control procedure is performed to vary the opening size of the flow control valve (the radiator flow) depending on the coolant temperature, such that the coolant temperature seeks a predetermined target value.
Thus, when the coolant temperature is relatively low, such as, if the engine has been started immediately before, the flow control valve is held in a fully closed state to warm up the engine quickly. Afterwards, when the coolant temperature rises to a relatively high level, feedback controlling is started such that the coolant temperature seeks the target value.
During the feedback controlling, if the opening size of the flow control valve falls in a range close to the fully closed state, or a relatively low opening size range, under a certain condition, the opening size of the flow control valve is adjusted in this range such that the coolant temperature seeks the target value. However, when the flow control valve is in the relatively low opening size range, the coolant temperature may change excessively with respect to the opening size adjustment of the flow control valve. This causes hunting in the coolant temperature, thus reducing the reliability of the feedback controlling of the flow control valve for adjusting the coolant temperature to the target value.
Also, as long as the opening size of the flow control valve remains in the relatively low range, changing of the radiator flow in response to the opening size adjustment of the flow control valve may become insufficient, depending on the flow characteristics of the flow control valve. For example, if the opening size of the flow control valve is decreased in the relatively low range by the feedback controlling to raise the coolant temperature to the target value, the coolant temperature does not rise sufficiently quickly. The opening size of the flow control valve is thus excessively reduced by the feedback controlling. In this case, if the engine operational state changes later such that the radiator flow, or the opening size of the flow control valve, must be increased, increasing of the opening size of the flow control valve is delayed. This causes overshooting of the coolant temperature, thus decreasing the reliability of the feedback controlling of the flow control valve for adjusting the coolant temperature to the target value. By contrast, if the opening size of the flow control valve is increased in the relatively low range by the feedback controlling to lower the coolant temperature to the target value, the coolant temperature does not drop sufficiently quickly. The opening size of the flow control valve is thus excessively increased by the feedback controlling. In this case, if the engine operational state changes later such that the radiator flow, or the opening size of the flow control valve, must be reduced, decreasing of the opening size of the flow control valve is delayed. This causes undershooting of the coolant temperature, thus decreasing the reliability of the feedback controlling of the flow control valve for adjusting the coolant temperature to the target value.
Further, in the feedback controlling of the flow control valve, delay is caused in the response of the radiator flow, or the coolant temperature, with respect to the adjustment of the opening size of the flow control valve. Such a delay decreases the efficiency for adjusting the coolant temperature to the target value by the feedback controlling. The controlling reliability of the coolant temperature with respect to the target value is thus decreased.